This invention is in the field of glass-plastic composites and more specifically relates to the production of high quality glass-plastic ophpthalmic lenses which, if desired, can provide photochromic properties.
The two primary advantages that plastic lenses have over glass lenses are those of lightness and tintability. Likewise, there are two primary advantages which glass lenses have over plastic; namely, glass is scratch resistant and can readily be given photochromic properties.
The mechanical durability of thin-glass-plastic composites is limited because of the expansion differences of the two components during temperature changes. This has long been recognized as a major problem in creating a successful composite. The mechanical stresses created by this expansion mismatch during periods of temperature changes causes glass breakage or delamination from the plastic. The coefficient of expansion of most glasses is below 9.times.10-6/C whereas the coefficient of expansion of most plastic lenses are in the neighborhood of 100.times.10-6/C. Additionally, the choice of adhesives between components is critical.
Burkley, in U.S. Pat. No. 3,508,987 (1970) discloses the utilization of elastomeric adhesive compounds between glass and plastic components that will accommodate the different expansion rates. Burkley, as in this invention, uses uniformly spaced components. In Burkley, the spacing is 0.3 to 0.4 mm. Ace, in U.S. Pat. No. 4,679,918, discloses an adhesive layer between a thin glass layer and an allyl diglycol carbonate layer to overcome an expansion mismatch during periods of reasonable temperature excursions (-40.degree. F. to 212.degree. F.). Sypcher, in U.S. Pat. No. 4,227,950 discloses a method of direct casting of allyl diglycol carbonate resin (ADC) onto a thin glass photochromic lens containing a uniform thermoplastic adhesive coating approximately 0.2 mm thick and, after curing the ADC, increasing the temperature above the melting point of the thermoplastic coating to adhere the elements together.
Another characteristic of glass is that it exhibits high resistance to fracture under compression stress, but a relatively low resistance to fracture under a tensile stress. As taught by Gulanti in U.S. Pat. No. 4,268,134 stresses in a glass layer at the edge of a glass-plastic composite lens consist of a meridianal force and a bending moment. The bending moment tends to fracture the glass layer near the lens edge.
Specifically, patent '918 uses an RTU silicone compound with elongation of 100% or greater and the specific examples given being 400% with a uniform thickness adhesive layer of 0.30 to 0.4 mm. The adhesive listed, UUU23-10, has a Shore A hardness of 20. It has been empirically determined that for an adhesive layer to be efficiently edged on conventional equipment at this thickness, a hardness of at least 69 Shore D is desirable. This is in order of magnitude much greater than a Shore A of 20. This lack of hardness presents a burden on production of such lenses. Skilled craftsmen are necessary to produce such lenses.
The prior art, therefore, has not found a completely satisfactory answer to bonding a thin glass lens to a plastic lens. Burkley and Ace have disclosed the use of a highly elastic bonding material that will move with the plastic lenses in periods of extreme temperature changes, but, in so doing, have created other problems that limits the use of such products. Spycher and Gulanti, although recognizing the problems, failed to rectify the problem in their respective approaches.